The present invention relates to an apparatus and method for starting and operating a brushless direct current motor, and in particular, starting and operating a brushless direct current motor without the use of add-on sensors. Brushless direct current motors are used in applications such as for disk drives and video cassette recorders and are typically under stringent requirements regarding their performance in these applications. Disk drive manufacturers are concerned with the possibility of having a backward rotation of the motor which may cause warpage of the read-write heads since they are often laying on the disk when it rests. This invention is directed to starting such a motor. Another requirement of users of these motors is that the torque of the motor is optimized once the correct direction of rotation during start-up is achieved. Failure to maximize the torque may cause losing control of the motor, ripple on the speed or loss of efficiency, which is not acceptable for applications, such as, portable, battery-operated systems.
The use of sensors such as Hall-sensors to detect the rotation of the motor, for example, as known in the art, often results in diminished performance of the motor with the offset of the Hall-sensor and errors due to the positioning of the Hall-sensor inside the motor.
Motor driving circuits utilizing the back-electromotive force sensing principle give an alternative to the use of sensors for motor driving circuits.
One such motor driving circuit based on the back-electromotive force sensing principle is disclosed in an article entitled "A Full-Wave Motor Drive IC Based on the Back-EMF Sensing Principle", by J. P. M. Bahlmann, IEEE Transactions on Consumer Electronics, Vol. 35, No. 3,Aug. 1989, pp. 415-420. The Bahlmann full-wave motor drive utilizes a bipolar integrated circuit which has been designed to allow full-wave driving of brushless three-phase DC motors. Three push-pull power stages are operated in a switching mode. The commutation moments are derived from the zero-crossings of the sensed back-electromotive force in the floating motor windings. A start-up system is also disclosed which generates commutations when no zero-crossing can be detected. Problems may occur, though in utilizing zero-crossing of the floating phase due to common mode signals especially during starting of the motor.